Piezoelectric device and method of



Jan. 18, 1938. A. L. w. WiLLlAMS v I v2,10(5,143

PIEZOELECTRIC DEVICE AND METHOD OF MAKING THE SAME Filed ApriL l, 1936 I IIIIIIIIIIIII/ m van .4 dam ma I Arm/m2? Patented J an. 18, .1938

UNITED STATES PATENT OFFICE PIEZOELECTI iiG DEVICE AND METHOD OF MAKING THE .SAME

Application April 1, 1936, Serial No. 72,149 In Great Britain April 5, 1935 1i Claims.

This invention relates to piezo-electric devices and particularly to the electrodes of piezo-electric elements and units and to the methods of forming such electrodes.

It is known (United States Patents Nos. 1,994,487 and 1,995,257) that the efilciency of a piezo-electric element is dependent, among other things, upon the relation of the element and its electrodes, the efficiency rising with increasing closeness or intimacy of the union between the element and the electrodes. This is particularly true of piezoelectric material such as Rochelle salt crystal, having large specific inductive capacity. Much success has been had by applying metal foil electrodes to Rochelle salt crystal elements by the methods disclosed in the aforesaid Patents 1,994,487 and 1,995,257, but the application of metal foil electrodes, particularly where the crystal elements are small and thin or where their surfaces are rough, presents diificulties in commercial production.

It has heretofore been proposed to apply a conductive coating of material such as platinum, silver, copper or other suitable conductive material to the surface of piezo-electric elements, particularly quartz elements, by sputtering, cathodic deposition, evaporation in a vacuum or chemical precipitation. It has also been proposed to provide internal electrodes in composite or so-called hour-glass Rochelle salt crystals by providing a cavity therein and filling the cavity with graphite, tinfoil, mercury or other suitable conductive material. It has further been proposed to form a coating of silver upon the internal pole surfaces of Rochelle salt crystals of the hourglass type by chemical precipitation. Also, it has been proposed to brush on or otherwise apply finely divided graphite in wet state to glass and other insoluble bodies, including bearing surfaces of machine elements. However, none of these methods as heretofore proposed is applicable to the electroding of Rochelle salt piezo-electric elements to achieve the purposes of the present invention, either because of the low melting point of Rochelle salt or its susceptibility to serious dehydration when placed in a vacuum or its solubility in water.

Accordingly one of the objects of the present invention is to provide a form of electrode and a method of forming the same characterized by exceedingly intimate contact between the piezoelectric element and the electrode and by great ease of formation and application of the electrode to the element regardless of the dimensions of the latter, or the smoothness of its surface.

A further object of the invention is to provide an electrode for piezo-electric elements which can be applied to the element with exceedingly close contact and which has adequate adherence to the element and adequate coherence to well adapt such elements for use in flexing multiple plate piezo-electric units in which electrode surfaces of the elements are firmly cemented together.

Another object of the invention is the provision of an electrode material that can be applied in Wet state to a soluble piezo-electric material such as Rochelle salt, without objectionable solution of the piezo-electric substance.

A further object of the present invention is to increase the efficiency of piezo-electric elements and of multiple element devices by providing an electrode for elements which can be more intimately joined to the element than electrodes heretofore employed.

In fabricating multiple-plate piezo-electric elements the preparation of relatively smooth surfaces on which to apply the electrode foil requires a very appreciable period of time. Accordingly another object of this invention is to provide an electrode for piezo-electric elements and devices that may be easily and quickly applied to relatively rough and uneven surfaces and also attain the intimacy of contact between the element surface and the electrode necessary for high efficiency.

The invention is based upon the discovery that finely divided materials suitably conductive for electrode purposes can be applied in a wet state tothe surface of a piezo-electric element of watersoluble piezo-electric material such as"R.ochelle salt in a particular manner as hereinafter described without significantly changing the homogeneous character of the water-soluble material and so as to produce a continuous electrode sheet or coating having extremely intimatecontact with the piezo-electric body and strong adherence thereto and also characterized by strong coherence and correspondingly great mechanical strength. In carrying out the invention, the finely divided electrode material preferably should have at least a substantial portion of its particles of colloidal dimensions. It is also preferable that the material should have a low specific gravity. The finely divided material can be applied to the piezo-electric elements in various ways but it is preferable to suspend it in a liquid vehicle and spray it on the element. Alternatively the powdered material can be moistened with a suitable liquid to form a thin paste or thick liquid which can be brushed upon the element.

Of the electrode materials available for the formation of the improved electrode, finely divided graphite has been found very suitable, graphite of colloidal dimensions being readily available commercially and such colloidal graphite being admirably adapted, by virtue of its fine particle size, relatively low specific gravity and adequate electrical conductivity, to serve the purpose of the invention. However, such electrodes can be formed of suitable metal, particularly relatively non-oxidizable metal, which may be suspended in a liquid vehicle and sprayed on the element.

A highly suitable graphite is available in the form of the commercial product sold under the registered trade-mark Aquadag. This product is graphite having particles of colloidal dimensions suspended in water and is understood to contain also a certain amount of protective colloidal material. This product is sold in the form of a cream or paste which, in practicing my invention, preferably is diluted by the addition of distilled water to give it a suitable consistency for use with an air brush or atomizer so that it can be sprayed upon the surface to be coated. In applying the coating in this manner to the surface of soluble piezo-electric material, such as Rochelle salt crystal, care should be used to avoid depositing too much water on the crystal. If it is attempted to produce a thick coating by continuous spraying, sufficient water may accumulate on the crystal body to dissolve enough of the Rochelle salt to produce coagulation of the graphite and, after recrystallization of the surface solution, cause substantial reduction of the piezo-electric eihciency of the device. This difficulty may be partially avoided in applying the electrode material by holding the crystal at some distance from the air brush so that some of the water has a chance to evaporate. In addition, it is desirable to spray the electrode material intermittently, giving an opportunity for excess water to evaporate between short spraying periods. Such evaporation of excess water can be hastened where an air brush is used by playing the air jet of the brush upon the surface under treatment. In proceeding in this manner, after the application of a light deposit to the crystal surface, that is to say, in sufiiciently limited amount to prevent significant solution of the crystalline material, the supply of solution is cut off and the air jet of the brush is played upon the surface of the crystal until any excess water has evaporated before applying more electrode material.

By proceeding in the preferred manner above described an electrode coating of controllable thickness can be formed on the crystal surface. Such a coating is characterized by a very intimate union with the crystal surface and by strong adherence to the said surface. The coating is also strongly coherent so that it may be built up to a desired thickness to give adequate conductivity over surfaces of wide area and still have adequate mechanical strength to permit the electrode surfaces of a plurality of the crystal elements to be cemented together to produce a flexing piezoelectric unit of the character disclosed in United States Letters Patent Reissue No. 20,213 or No. 1,803,275.

Instead of building up the thickness of the electrode by application of finely divided electrode material until the desired conductivity of the electrode is secured, it is possible to attain a similar result by applying to the piezo-electric element a thin or moderate coating of the finely divided material and then cementing to such coating 9. suitable metal foil or metal gauze.

In many instances it is desirable that the electrode have a lead extension to provide for suitable electrical connections. To this end it is preferable to attach a strip or strips of suitable conducting material such as metal foil to the electrode by cementing it thereto or by embedding it partially therein or by both cementing and embedding it. Thus, for example, after a thin coating of the finely divided electrode material has been applied to the piezo-electric element the end of a lead extension can be cemented thereto and then the thickness of the electrode can be built up by the application of more of the finely divided material resulting in the embedding of a portion of the electrode lead. Alternatively, after the application of a thin coating of the finely divided material to the piezo-electric element the electrode lead can be cemented thereto and a sheet of metal foil can then be cemented to the electrode so as to cover a portion of the lead, and such foil sheet may extend fully or partially over the area of the electrode.

For a better understanding of the method and of the improved devices which result from the use of the method, specific procedure in connection W with particular forms of piezo-electric devices will now be described, by way of example, with reference to the accompanying drawing.

In the drawing, Fig. 1 is a perspective view showing mask devices for holding pieZo-electric elements while electrode coatings are applied thereto by spraying.

Fig. 2 is a perspective view of a plate-like piezoelectric element.

Fig. 3 is a similar view after an electrode of finely divided material has been applied thereto.

Fig. 4 is a similar view showing the electroded element of Fig. 3 with an electrode lead extension applied.

Fig. 5 is a view similar to Fig. 4 showing the same element after the electrode of finely divided material has been built up in thickness over a portion of the lead extension.

Fig. 6 is a perspective view of a form of construction alternative to that shown in Fig. 5, the electrode substance overlying the lead extension being in the form of a metal foil cemented on.

Fig. '7 is a perspective view showing a modification of the construction in Fig. 6, in which the metal foil is coextensive in area with the electrode.

Fig. 8 is a side elevation of a multiple-plate piezo-electric unit of the fiexing type of which the elements are provided with electrodes in accordance with the present invention.

Fig. 9 is an expanded edge elevation of the unit shown in Fig. 8.

Fig. 10 is a side elevation of one of the electrode leads shown in Fig. 8.

Fig. 11 is a fragmentary sectional view on a greatly enlarged scale, the section being taken on the line li--ll of Fig. 8.

Fig. 12 is a side elevation of a multiple-plate flexing piezo-electric unit of modified construction.

Fig. 13 is an expanded edge elevation of the unit shown in Fig. 12.

Fig. 14 is a fragmentary section on a greatly enlarged scale, the section being taken on the line l4-l4 of Fig. 12.

Referring to the procedure illustrated in Fig. 1, I and 2 are metal sheets formed with registering openings such as la, the openings being somewhat smaller than the dimensions of the plate-like piezo-electric elements to be electroded. Such elements 3 are secured between the sheets I and 2 by the clamps 4, 4 in a manner to support the elements and expose the major parts of their faces while masking marginal portions thereof adjacent all their edges. With piezo-electric elements of Rochelle salt, ior example, secured by masks in this manner, electrode coatings are readily applied by holding the assembly before an atomizer or air brush nozzle 5. The procedure in applying the finely divided electrode material, such as colloidal graphite, is as above described. After coating the elements on one side their other sides may be similarly treated, if desired, by simply reversing the mask plate assembly so as to subject its other side to the spray of the air brush.

By this procedure a plate-like element 6 such as is shown in Fig. 2 and preferably formed of homogeneous Rochelle salt crystal may have an electrode of colloidal graphite applied thereto as shown at I in Fig. 3.

After the electrode 1 has been applied as shown in Fig. 3, an electrode lead extension 8 may be formed by cementing a strip of metal foil or the like to the surface of the finely divided electrode material as shown in Fig. 4.

To increase the conductivity of the electrode, additional finely divided electrode material can be sprayed upon the element as shown at 9 in Fig. 5 so as to cover and embed a portion of the lead 8.

A modification of the construction 01 Fig. 5 is shown in Fig. 6 in which an electrode of finely divided material To is applied to a piece-electric plate 6a, an electrode lead extension 8a is cemented to the electrode coating 1a and then a sheet 9a of metal foil is closely cemented over the electrode coating la and a portion of the extension lead 811.

In Fig. 7 a modification of the construction of Fig. 6 is shown in which the metal foil layer of the electrode is in the form of a sheet 9b which is coextensive in area with the finely divided coating M of the electrode.

The types of electrode construction illustrated in Figs. 5, 6 and 7 are preferable to that shown in Fig. 4 as the electrodes of the former constructions insure better connection between the electrode lead extensions and the bodies of the electrodes and in addition, particularly in the case a construction shown in Fig. 7, the electrode a more adequate conductivity from point throughout its area without undue ne graphite coating.

nose of further illustrating the h the invention may be applied there is shot n Figs. 8 to 11, inclusive, 2. piezoelectric unit of the multiple-plate flexing type, such as is disclosed in United States Patent 1,802,782 or 1,803,275, embodying the improved type of electrode construction. As is best shown in the expanded edge elevation of Fig. 9, the unit comprises two plates H], it cut from a homogeneous Rochelle salt crystal, each of which plates carries upon its two faces electrodes comprising colloidal graphite coatings l2, l2 of the character above described. The two plates hearing these coatings are cemented together with an electrode extension l3 clamped between them as shown in Figs. 8 and 9. The electrode extension i3 is formed of very thin sheet metal, one end of which is struck up or. crimped to form a series of teeth I311 which. as shown in the enlarged view of Fig. 11, are adapted to penetrate the cement it which unites the two plates l and H and so insure electrical contact of the extension IS with the graphite coatings l2. Another electrode extension II is partially split lengthwise and its two branches or arms Ila, b are cemented to the graphite coatings II on the outer faces of the unit and over the parts Ha, Mb are cemented -metal foil elements l5, l which serve to eifect a more effective connection between the graphite coatings l2 and the extension It. It will be understood that in the drawing, .and particularly in Fig. 9, in the interest of clarity, the parts are not drawn strictly to scale.

Figs. 12, 13 and 14 show a modified form of multiple-plate unit embodying the invention. In this construction the piezo-electric plates l1, l8 of Rochelle salt crystal are provided with graphite coatings l9, [9 on their two faces and the elements thus formed are firmly united by cement 23, in this case without any electrode extension from the inner electrodes. To insure electrical connection between the internal graphite coatings l9, I! of the unit, it is preferred todistribute powdered or finely granulated conducting material lQa over one of the graphite coatings before the two plates are cemented together, or to mix such granular material in the cement 23. The minute granules of this material, which may be of silver or some other good conducting material, penetrate the film of cement and serve to make effective electrical connection between the two graphite coatings, as indicated in Fig. 14.

To the outer electrode coatings l9 are cemented electrode extensions 26 and 2|, respectively, and over these extensions are cemented metal foil strips 22, 22 which serve the same purpose as the strips l5, l5 of the first described unit.

The toothed or roughened electrode extension I3 of the construction shown in Figs. 8 to 11 and the minute conducting granules Illa of the construction shown in Figs. 12-14, by providing effective electrical connections between the internal electrodes make it unnecessary forthe electrostatic field of the unit to pass through the film of cement between the crystal plates so that said field is effectively utilized in the piezo-elec tric plates and at the same time the advantage of extremely close or intimate contact between the electrodes and the plates is attained.

The intimate union between the improved electrode and the crystal surface results in very high specific inductive capacity. Numerous tests of Rochelle salt crystal elements electroded as described above show that the values of specific inductive capacity obtained with this improved form of electrode may run as high as 90% of the values obtained when aqueous electrodes are employed. These values are obtained when the improved electrodes are applied to crystalline surfaces that are free from grease, dirt and other impurities, including dehydrated Rochelle salt. Average values of capacity, working under commercial production conditions, run about 70% of the wet electrode values. The use of wet electrodles may be considered to result in the maximum possible value of capacity obtainable due to the extremely intimate union between the liquid electrode and the crystal surface.

The very strong adherence and coherence of the sprayed graphite coating render electrodes formed in the manner stated highly suitable for piezo-electric plates of the flexing type. This is true not only where the electrodes are on expcsed surfaces of the crystalline bodies but also where they are on the adjacent surfaces of crystalline plates that are cemented together to form a multiple-plate unit. Indeed the advantage is perhaps the more striking in the latter cases. Tests of Rochelle salt multiple-plate units having the sprayed graphite electrodes cemented together have shown that the shear strength of the unit on lines parallel to the plates is from three to five times the strength of similar units having metal foil electrodes. In the case of the latter units, all samples tested failed at the union of the foil and the crystal to which it was cemented, whereas in the case of the units with graphite electrodes, if the electrodes were not made thicker than necessary to afford the desired conductivity, the failure in all cases was in the crystalline material and not in the electrode joint. In cases where the graphite electrode was made much thicker than necessary the unit failed in shear in much the same way as the units with metal foil electrodes. In other words, if the graphite electrodes sprayed on Rochelle salt crystal plates are not made much thicker than necessary to give the desired conductivity, a cemented union between the electroded plates of superior strength is secured. It has been found that a sprayed graphite electrode of 0.001" thick has ample conductivity for practically all purposes and that such electrodes less than 0.0005" thick are suitable for most applications. The relative thinness of the new graphite electrode permits of their formation in relatively short time and so facilitates production. It also permits the production of thin units with a minimum thickness of piezo-electrically inactive material. The great strength of the joint between plates with the graphite electrodes is an advantage in all cases but is greatest perhaps in cases where the margin between the edge of the electrode and the edge of the crystal plate is quite small or nil and the strength of the joint between the electrodes of two plates must be relied upon to hold the plates together.

In applying metal foil electrodes by previous manufacturing practice very rigid control of cementing conditions and considerable skill in application were necessary to insure permanent adhesion between the metal foil electrodes and the crystal surface. The electrode provided by this invention is applied directly to the crystal surface without the use of a separate adhesive cement and its application to crystal sections of any practical size requires relatively little skill. The new type of electrode is well adapted to be applied by automatic devices.

Furthermore, in previous practice, in order to attain the intimacy of contact between the electrode and crystalline surface necessary for reasonable efficiency it had been found advantageous to finish machine the crystal surface produced by sawing before applying the electrode. In the present invention, due to the fine particle size of the improved electrode material as it reaches the crystal surface, and the gradual increase in thickness of the deposit during application, the resulting electrode surface conforms to the crystal surface and maintains intimate contact regardless of the degree of smoothness of the crystal surface. Thus electrodes of high emciency may readily be applied to unmachined crystal slabs or to crystal surfaces of other than planar character.

In many applications of piezo-electric crystal elements it is desirable that the electrodes be limited in size and shape or that more than one separate electrode be applied to a crystalline surface. The area of crystalline surface covered by the electrode deposit may be controlled by providing a mask, as above explained or otherwise, for the crystal section which shields the portions that are to remain uncoated.

.As has been indicated, the improved electrodes can be formed of finely divided materials other than graphite and the consistency of the electrode material and the methods of applying it may vary. For example, the Aquadag paste or cream can be thinned with alcohol instead of distilled water. Again, the undiluted Aquadag paste or cream can be applied to the crystal surface with a brush. Similarly mixtures of different finely divided materials may be easily employed, or successive coats of different materials applied.

In lieu of the Aquadag or other colloidal graphite preparations, finely ground commercial graphite powder, or mixtures of the colloidal graphite and such commercial graphite powder, suspended in a solution of alcohol, benzol, water or other liquid can be employed. Such commercial graphite coating can be improved by adding a suitable protective colloid, fixative or binder to the suspension. However, the use of the colloidal graphite is preferable. Instead of graphite, finely divided metal may be used. For example, a suspension of colloidal gold may be sprayed upon the crystal surface.

Electrodes formed of colloidal graphite or other finely divided material sprayed on in the manner described can be applied with relatively great ease and rapidity to crystal elements of any size, including very small and exceedingly thin elements; and coatings of any shape or pattern may readily be formed by masking portions of the crystal surface.

What I claim is:

1. A method of making piezo-electric devices comprising bodies formed of homogeneous watersoluble crystalline material of high specific inductive capacity and having electrodes on one or more surfaces thereof, said method comprising applying directly to the body surface to be electroded a thin coating of conductive material in a wet state and in sumciently limited amount to prevent significant solution of the said crystalline material and sufficiently finely divided to adhere, after drying, to the piezo-electric material in intimate contact therewith; causing the wet coated surface to dry; and thereafter applying additional conductive material to the coating to increase its conductivity.

.2. A method of making piezo-electric devices as in claim 1, in which the additional conductive material is finely divided and applied in the same manner as that first applied in forming the electrode.

3. A method of making piezo-electric devices as in claim 1, in which the additional conductive material is in part at least in the form of metal foil applied to the coating of finely divided material by cementing it thereto.

4. A method of making piezo-electric devices as in claim 1, in which the finely divided electrode material is at least in part in the colloidal state.

5. A method of making piezo-electric devices as in claim 1, in which at least a part of the finely divided electrode material is colloidal graphite.

6. A method of making piezo-electric devices as in claim 1, in which the piezo-electric material has substantially the piezo-electric properties of Rochelle salt crystal.

7. A method of making piezo-electric devices as in claim 1, in which the finely divided electrode material is intermittently sprayed in liquid suspension upon the piezo-electric body with the spraying periods sufilciently short and separated by intervals of suilicient length for drying to avoid solution of the piezo-electric material.

8. A piezo-electric device comprising in combination a body formed of homogeneous watersoluble piezo-electric material having a high dielectric constant and an electrode coating on a surface of the body comprising finely divided material having particles thereof in direct intimate contact with and adherent to the piezo-electric material of the body.

9. A piezo-electric device as in claim 8, in which the piezo-electric material of the body has a dielectric constant of the same order 01' magnitude as that or Rochelle salt crystal.

. 10. A piezo-electric device as in claim 8, in which the piezo-electric material of the body is Rochelle salt crystal.

11. A piezo-electric device as in claim 8, in which the electrode coating is formed in part. at least of conducting material in the colloidal state.

12. A piezo-electric device as in claim 8, in which the finely divided material of the electrode coating consists in part at least of colloidal graphite.

13. A piezo-electric device as in claim 8, in which the electrode comprises a thin coating of finely divided conducting material having particles thereof in direct intimate contact with and adherent to the piezo-electric material of the body and a sheet of metal foil firmly cemented to said thin coating and serving to increase the conductivity oi. the electrode.

14. A piezo-electric device as in claim 8, in

which the electrode comprises a lead extension a portion or which is embedded in the electrode.

15. A piezo-electric device as in claim 8, in which the body of the piezo-electric material is formed exteriorly with two opposite parallel surfaces and in which each of said surfaces has an electrode coating of the character specified.

16. A method of making piezo-electric devices comprising bodies formed 01' material oi. high specific inductive capacity and having electrodes on one or more surfaces thereof, said method comprising applying directly to the body surface to be electroded a conducting material in a sumciently finely divided state to adhere to the piezoelectric material in intimate contact therewith and so as to form a thin electrode coating, and thereaiter applying to the coating to increase its conductivity additional conductive material, including metal foil cemented to the finely divided material.

1'7. A piezo-electric device as in claim 8 in which the electrode comprises a thin coating oi. finely divided conducting material having particles thereof in direct intimate contact with and adherent to the piezo-electric material of the body, a sheetof metal foil firmly cemented to said thin coating and a lead extension a portion of which is embedded in the electrode between the finely divided material and the metal foil, the sheet metal foil forming a connection of high conductivity between the lead extension and the finely divided conducting material of the electrode.

ALFRED L. W. WILLIAMS.

CERTIFICATE OF CORRECTION.

Patent No 2,106,115

January 18 1958 ALFRED L. W. WILLIAMS.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows; Page 5, first column, lines 59 and 60, for "United States Patent 1,802,782 or 1,805,27 5" read United States letters patent Reissue No. 20,213 or No. 1,805,275; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 8th day of March, A. D. 1958.

(Sea-1) Henry Van Arsdale,

Acting Commissioner of Patents. 

